The present invention relates generally to micromechanical resonators, and more particularly, to multi-frequency high-Q tunable micromechanical resonators.
In Asia, Europe, and North America, multiple frequency standards and infrastructures have been established with which cellular handsets must comply in order to provide connectivity for users. Thus, multi-band cellular handsets are required. Currently, the frequency standards in use are nominally 850, 900, 1800, and 1900 MHz. The latest technology also requires filters at 2.4 GHz and 5 GHz. Although purely digital WCDMA networks are evolving across the globe, analog protocols at 850 and 900 MHz must also be supported for connectivity is less-developed areas. In the United States, the FCC mandates freeing the currently occupied 300-800 MHz analog (video) broadcast bands may enable mobile communications over this spectrum. For these reasons, cellular handsets will also require analog band and low-UHF compliance for years to come. Piezoelectric thin film resonators, which can be subcategorized into solidly mounted resonators and film bulk acoustic resonators, have been proposed for traditional surface acoustic wave filter replacement. However, since thin film resonators utilize the thickness dilation of a thin deposited film, obtaining resonators with widely dispersed frequencies would typically require film deposition at multiple thicknesses. The piezoelectric films are typically sandwiched on both sides with metal electrodes. One technique to provide frequency variance, although difficult to implement, is provided by selective deposition of metal electrodes. However, obtaining the 2:1 frequency ratio between 900 MHz and 1800 MHz is not feasible.
Independent groups have demonstrated bandpass filters at 900 MHz and 1900 MHz using thin film resonators. The 1900 MHz film bulk acoustic resonator filters, which are integrated with electronics, are currently in production for handsets which use a US-based PCS network. However, filter solution at multiple frequencies on a single die or single package are not known to exist. For this reason, single-chip integrated thin film resonator-based bandpass filters have not penetrated markets that utilize multiple frequency standards such as Asia and Europe.
With increasing demand for higher level of integration in existing electronic systems and emerging applications, alternatives to bulky frequency selective components and resonant sensors are necessary. Micromechanical resonators are choice candidates owing to their small size and ease of integration. Several demonstrations of capacitively transduced, silicon micromechanical resonators with high Q have been demonstrated. Typical capacitive UHF resonators require large polarization voltages and ultra-thin electrode-to-resonator gap spacing to achieve motional impedances (R1) less than 1 kohm. These two requirements pose additional demands on resonator fabrication and interface circuits. In contrast, piezoelectric resonators can be fabricated with relative ease using low temperature processes and have lower R.sub.1 due to greater coupling.
Examples of piezoelectric resonators include quartz crystal units, surface acoustic wave (SAW) resonators and thin-film bulk acoustic resonators (FBAR). The main drawbacks of crystal units and SAW devices are their bulky size and incompatibility for microelectronic integration. On the other hand, FBARs can be integrated with on-chip electronics and have been demonstrated at GHz frequencies. Since FBARs utilize the thickness vibration of a thin film, obtaining multiple dispersed frequency standards on a single substrate is challenging.
It would be desirable to have multi-frequency high-Q tunable micromechanical resonators for use in multi-band cellular handsets, and the like. It would also be desirable to have composite bulk acoustic resonators implement dispersed-frequency devices simultaneously on a single substrate.